Bradford had heard about Anne Hutchinson ('Huchingson'), who challenged the authority of the Puritan ministers. He was afraid she would move to Plymouth. She would soon be banished, and settle with her followers in Portsmouth in what became the Colony of Rhode Island and Providence Plantations.
Bradford's question about 'a monstrous birth' alludes to Mary Dyer, a follower of Hutchinson who gave birth to a deformed stillborn child. Hutchinson had been her midwife and tried to hide the birth. Winthrop and Hutchinson's other enemies viewed the birth as a punishment from God.
Here's the letter:
I thanke you for your letter touching Mrs Huchngson; I heard since of a monsterous, & prodigious birth which she should discouer amongst
you; as also that she should retracte her conffession of acknowledgmente of those errours, before she wente away; of which I have heard many various reports. If your leasure would permite) I should be much behoulden unto you, to certiffie me in a word or tow, of the trueth & forme of tht monster &c.
To read the whole letter, and an explanation from the Massachusetts Historical Society, click here.